1. Field of the Invention
This invention relates generally to a laser beam delivery system suitable to deliver the output of an infrared laser device to a remote location through a flexible fiber optic cable. More particularly, it relates to a laser beam delivery system that uses a hollow-core photonic band gap (HC-PBG) fiber made of a non-silica-based glass to transmit a high-power or high-energy laser beam while preserving the high laser beam quality at the system output.
2. Description of the Prior Art
The use of fused silica glass fiber in fiber optic laser beam delivery systems to deliver laser radiation has been demonstrated in the early 1980's. In particular, silica fiber with large core diameter of 50–1000 μm is being commonly used to transmit several watts to several kilowatts of laser power for applications like welding, machining, marking or laser surgery. For example, U.S. Pat. No. 4,676,586 describes one such fiber optic laser beam delivery system for high power transmission. But large core silica fiber cannot currently be used for high beam quality processes because of the highly multimode nature of these fibers. Furthermore, as is known, fused silica fiber is limited to transmission in the wavelength range of about 0.2 to 2 μm, and cannot be used in laser beam delivery outside this range.
There are many useful lasers that operate at wavelengths above 2 μm in the infrared region. Among these, there is the Er:YAG laser (˜10 W at 2.94 μm) and the Cr, Er:YSGG laser (˜10 W at 2.78 μm) which are used for medical and dental applications. Also, CO2 lasers emit 10 to 10,000 W at 10.6 μm and are used mainly in industrial applications, but also in medical applications.
Many optical fibers have been developed to transmit infrared laser power radiation at wavelengths beyond 2 μm. Thus, germanate glass fiber based on GeO2 is manufactured by Infrared Fiber System (Silver Spring, Md., USA) and transmits from 1 to 3 μm. Large core and highly multimode germanate fiber (150–500 μm) has been used to transmit up to 20 W (2J at 10 Hz) of Er:YAG laser. Single crystal sapphire optical fiber is another commercially available infrared fiber which is manufactured by Photran (Poway, Calif., USA) and is transparent from the visible to 3.5 μm. Sapphire fiber growth method is slow (few mm/min) and produces only a fiber with a large diameter of 150–450 μm and a length of less than 2 m. The sapphire fiber core is highly multimode and has been used to deliver over 10 W of average power from an Er:YAG laser operating at 2.94 μm.
Hollow glass waveguide (HGW) is another infrared transmitting fiber. HGW fiber was developed at Rutgers University, Piscataway, N.Y., USA and is disclosed in U.S. Pat. No. 5,440,664. HGW fiber is fabricated using wet-chemistry methods to deposit a dielectric-enhanced metal layer inside silica glass tubing. The silica glass tubing used has a polymer or polyimide coating on the outside surface to preserve mechanical strength. HGW fiber has been used for high-power CO2 laser transmission (see R. K. Nubling and J. A. Harrington, “Hollow-waveguide delivery systems for high-power, industrial CO2 lasers”, Appl. Opt., vol. 34, No. 3, 20 Jan. 1996, p. 372–280). HGW fiber offers two main advantages because of the air guiding core: first it allows high laser induced damage threshold (LIDT), namely more than 1000 W of CO2 laser; and second it has very low insertion loss because there are no Fresnel reflection losses at the ends of the HGW fiber. However, transmission losses of the HGW fiber vary as 1/a3 where a is the bore radius and, therefore, the loss can be arbitrarily small for a sufficiently large core (300–1000 μm). Another disadvantage of the HGW fiber is the added loss on bending which is also polarization sensitive. This means that the output power as well as the output beam profile vary significantly with different bending radius of the HGW fiber and/or polarization state of the input beam.
All of the above mentioned infrared delivery fibers offer some viable solutions and some drawbacks for the transmission of high power infrared lasers. But none of them can transmit efficiently stable high power infrared laser radiation with singlemode beam quality.
During the past ten years, researchers have developed a new class of optical fiber called photonic crystal fiber (PCF), also known as microstructured or holey fiber, which can combine large core dimension and Gaussian-like fundamental mode transmission. Broadly speaking, PCF may be defined as a fiber having a core surrounded by a periodically cladding structure, in which the periodic variation appears in directions perpendicular to the fiber axis, whereas the structures are invariant along the fiber axis. Of particular interest is the hollow-core photonic band gap (HC-PBG) fiber where the light is guided in a hollow core surrounded by a periodically cladding structure. Silica glass HC-PBG fiber was first disclosed in 1995 (Full 2-D Photonic Bandgaps in Silica/Air Structures, Birks et al., Electronics Letters, Vol. 31 (22), p. 1941–1943, 26 Oct. 1995). Crystal Fibre of Birkerød, Denmark manufactures a silica HC-PBG fiber to guide light in air and to deliver very high laser peak power while preserving an almost perfect output beam quality. But silica glass HC-PBG fiber does not transmit well beyond 2 μm since several percent of the light propagates in the periodically cladding region and this light is highly attenuated through absorption in silica.
Recently, researchers at the Naval Research Laboratory, USA, have developed a novel HC-PBG fiber. U.S. Patent Application Publication No US 2005/0074215 A1 discloses a HC-PBG fiber made of non-silica-based glass such as chalcogenide, germanate, phosphate, tellurite, borate, antimonate and halide to transmit infrared light. The non-silica-based glass is a low melting temperature glass (˜300° C.) and can transmit light from 1 to 15 μm depending on the composition. This infrared transmitting HC-PBG fiber features a hollow core surrounded by an array of air holes in the non-silica-based glass periodically cladding structure. The surface of the holes in the periodically cladding region surrounding the hollow core can be very thin and very easy to damage. Hence the combination of low-melting temperature of the non-silica-based glass material, of fragile thin holes surface and of high power laser, make the non-silica-based glass HC-PBG fiber practically impossible to use directly to deliver high power infrared laser.
Another type of HC-PBG fiber is disclosed in U.S. Pat. No. 6,898,359 wherein the periodically cladding region is arranged concentrically around the hollow core and the photonic band gap effect is provided with successive layers of high and low index of refraction. This structure can also be designed to transmit infrared light ranging from 2–12 μm, provided a suitable delivery system can be designed.
Consequently, there is a need for a delivery system that would make use of non-silica-based glass HC-PBG fiber for transmission of high power laser in the infrared.